Hydrogen sulfide (H2S) is commonly found in oil reservoirs due to its production by sulfate-reducing bacteria (SRB), which may be indigenous to an oil reservoir and/or introduced such as during water injection in water flooding secondary oil recovery methods. The metabolism of these SRB converts sulfate that is typically present in injection water to sulfide, which results in souring of a reservoir and the oil produced, thereby reducing the value of the recovered crude oil. In addition sulfide in production water causes corrosion of equipment used to recover oil including storage reservoirs, surface facilities, and pipelines, and it can cause plugging by the formation of iron sulfide, as well as causing health and environmental hazards.
In oil reservoirs and in production and injection fluids either or both of SRB and nitrate-reducing bacteria (NRB) may be present, either as indigenous populations or through introduction. When both are present, there may be competition for nutrients between SRB and nitrate-reducing bacteria (NRB). The presence of SRB and NRB, the presence and types of nutrients available, as well as the balance of sulfate, nitrate, and nitrite are all factors affecting levels of sulfide in the reservoirs and fluids.
One method used to reduce sulfide has been to add nitrate to injection water that is administered field-wide to an oil reservoir through multiple injection wells (Griroryan et al. 2009 J. Can. Petrol Technol. 48:58-61). Injection of water containing nitrate has been tested in continuous or pulsed applications, and when introduced to a portion of a limited section of a reservoir, using nitrate at 150 ppm to 40,000 ppm (Voordouw et al. (2009) Environ. Sci. Technol. 43:9512-9518).
U.S. Pat. No. 5,405,531 discloses removing H2S and preventing SRB production of H2S in an aqueous system by introducing nitrite and nitrate and/or molybdate ions in concentrations where denitrifying microorganisms outcompete SRB for available nutrients. Generally less than about 3000 ppm of total nitrate and nitrite ions is added to the aqueous system that is then injected into an oil-bearing formation, more particularly between about 25 and 500 ppm. U.S. Pat. No. 7,833,551 discloses inhibiting sulfide production by SRB by treating SRB with a non-oxidizing biocide and a metabolic inhibitor, which requires lower concentrations of biocide and inhibitor than when each is used alone.
There remains a need for additional effective methods to reduce sulfide in production fluid.